2011 QCD and High Energy Interactions - Rencontres de Moriond ...

Figure 1: Helix string parametrizations. Left: original proposal (Eq.1). Right: modified helix scenario (Eq.2).
The lines in the string area diagram connect points with identical helix phase.
A modified version of the helix string scenario has been proposed in 3 , with helix phase
difference proportional to the amount of energy stored in the QCD string ( in the string rest
frame )
∆Φ = S κ ∆l (2)
where S is a parameter, κ is string energy density, and ∆l is distance (along the string) separating
points with helix phase difference ∆Φ.
The difference between models is illustrated by Fig. 1. In the original helix string scenario,
the helix winding becomes faster towards string endpoints, while the modified scenario defines
a regular, static helix structure.
3 Experimental signature
The modified helix string fragmentation scenario (Eq.2) predicts a number of features which
can be observed experimentally. The size of transverse momentum pT of a direct hadron is
correlated with its energy E ( in the rest frame of the string) via relation
| pT | = 2 r|sin(0.5 S E)| (3)
where r stands for radius of the helix structure. The correlations are shown in Fig. 2 (left) for a
toy case of simple hadronic system without hard gluon kinks. Fig. 2 (right) shows the difference
between the standard pT modelling ( via tunneling ) and the helix driven pT modelling in the
inclusive pT spectrum. Direct observation of E−pT correlations is difficult due to the smearing by
parton shower, but the qualitative difference in the shape of the inclusive spectrum is preserved.
Interestingly enough, such type of discrepancy is readily observed in hadronic data 4,5 .
The parameters of the helix string model have been adjusted using LEP data with help of
Rivet/Professor tools 6 . A simultaneous tuning of 6 parameters (including helix radius r, pitch
S, Lund parameters (a,b), parton shower cut-off and effective coupling constant ΛQCD) has been
performed for two parton shower algorithms (Pythia pT ordered parton shower 7 and Ariadne
parton shower 8 ) interfaced to private implementation of the helix string fragmentation algorithm